The present invention relates to an asymmetric porous polytetrafluoroethylene membrane for a filter. Also, the present invention relates to material for a filter comprising the asymmetric porous polytetrafluoroethylene membrane for a filter and a reinforcing material.
Porous polytetrafluoroethylene membrane (hereinafter referred to as porous PTFE membrane) has excellent chemical resistance and high tensile strength and therefore is suitably used for a variety of purposes such as a filter for filtering gas and liquid, an agent for an air permeable and water impermeable membrane for clothing and a sheet for medical use, in addition to a sealing or gasket for piping and production facilities in the fields of chemicals, foods and semiconductors.
A process for preparing a porous PTFE membrane is disclosed (for example see U.S. Pat. No. 3,953,566, U.S. Pat. No. 3,962,153, U.S. Pat. No. 4,096,227 and U.S. Pat. No. 4,187,390), in which PTFE paste, which is a mixture of PTFE fine powder and an extrusion aid such as naphtha, is extruded and then rolled. Then, after removing the extrusion aid from the rolled article, the article is drawn in a uniaxial or biaxial direction. Subsequently, to maintain the shape of the drawn porous PTFE membrane, heat-setting is conducted at a temperature between 35° C. and the melting point of PTFE.
Also, many documents disclose using the obtained porous PTFE membrane as a filter but the porous PTFE membrane disclosed in these documents have the problem that air permeability and collection efficiency is insufficient (for example see U.S. Pat. No. 5,234,739, U.S. Pat. No. 5,395,429 and U.S. Pat. No. 5,409,515).